The present invention relates to improvements in security substrates incorporating a security feature which inhibits the ability of counterfeiters to produce counterfeit notes which is formed from at least two layers which are indistinguishable in reflected light but are distinguishable in transmitted light.
The increasing popularity of colour photocopiers and other imaging systems and the improving technical quality of colour photocopies has led to an increase in the counterfeiting of banknotes, passports, and identification cards etc. There is, therefore, a need to add additional authenticating or security features to existing features. Steps have already been taken to introduce optically variable features which cannot be reproduced by a photocopier into such documentation. There is also a demand to introduce features which are discernible by the general public but which are “invisible” to, or viewed differently, by a photocopier. Since a photocopying process typically involves scattering high-energy light off an original document containing the image to be copied, one solution is to incorporate one or more features into the document which have a different perception in reflected and transmitted light, one example being watermarks and enhancements thereof.
US-B-4,307,899 describes an identification card with hallmarks which are adapted to be inspected in both transmitted and incident light. This comprises a homogenous white layer printed on a transparent substrate with a layer printed thereover which has gaps in the form of indicia. A further layer is printed on top of this layer in the form of the indicia, but offset with regard to the gap. When viewed in transmitted light, the single layer printed portions appear bright, whilst in incident light they appear dark. The multilayer printed portions, on the other hand, appear dark in transmitted light and bright in incident light, thus producing a watermark like effect.
EP-A-0657297 describes a security document which uses light interference pigments in a layer printed over a transparent support, over which is printed a pattern containing a common light reflecting pigment. This provides a security feature which cannot be copied by photographic techniques. By changing the viewing conditions from transmission to reflection mode, the differently printed parts change their colours complementarily, so that the colours become inverted.
Another type of security feature which cannot be reproduced by a photocopier is one which has different perceptions at different viewing angles in reflected light, an example of which is described in EP-A-1592561. In the method described in this patent specification a motif is printed on to a substrate, over the top of which is printed a semi-transparent motif using an optically variable ink. This enables the background on which it is printed to be visible in reflected light, whilst allowing the overlying motif to be seen at varying angles. This represents a combination of three superimposed colours, being the background colour and the two colours of the optically variable ink.